The object of the present invention is a method for setting the operating frequencies of a multiband mobile telephony telephone and a telephone thus obtained. It can be used chiefly in the field of mobile telephony when it is necessary to go from a given frequency band to another given frequency band, in most cases according to another standard for the encoding of transmitted messages. The aim of the invention is to reduce the cost of manufacture of a multiband telephone of this kind.
In the field of radiocommunications, there is a known standard called the GSM standard in which the messages are broadcast by base stations and/or mobile telephones in the 900 MHz range. Furthermore, there is the known DCS standard in which the frequency range is about 1800 MHz. There also exists the PCS range in which this frequency band is 1900 MHz. There is also the UMTS standard in which the frequency band is 2200 MHz. The existence of all these bands or other future ones of course favours the multiplicity of communication networks and, hence, a greater possibility, in terms of frequencies, of users being connected with one another.
In addition to different frequency bands, there are transmission channel division modes of the time, frequency or coding type such that it is possible to convey various, simultaneous communications from one and the same base station and in one and the same geographical domain. The transmissions thus are of the TDMA, FDMA or CDMA type or, again, of the combined type. These terms signify Time Division Multiple Access, Frequency Division Multiple Access and Coded Division Multiple Access, translated in French as Accxc3xa8s Multiple à Rxc3xa9partition dans le Temps, Accxc3xa8s Multiple à Rxc3xa9partition en Frequence, or Accxc3xa8s Multiple à Rxc3xa9partition de Codage, AMRT, AMRF, and AMRC. The invention can be applied whatever the mode of transmission from this viewpoint.
The instruments referred to are obviously transmitters-receivers and must comprise a transmission and a reception chain. The base stations often comprise, in fixed positions, the same blocks and components as mobile telephones since the latter are mass produced and therefore inexpensive. For this reason, the invention relates to mobile telephony telephones whether these telephones are themselves mobile or not.
In such telephones, on the basis of a common antenna, a device is used to separate a reception channel from a transmission channel. In the transmission channel, given a frequency agility that is generally implemented in methods of mobile telephony, it is the common practice to make a transmitter with three oscillators. A first oscillator, which is an output oscillator, produces a signal at a transmission frequency. This transmission signal is furthermore mixed in a mixer with a signal produced by an oscillator producing a signal at a transition frequency. The mixer produces a difference signal having a frequency that corresponds to the difference between the transmission frequency and the transition frequency. This difference signal is then compared with a signal produced by a third oscillator at intermediate frequency. To put it in precise terms, the intermediate frequency produced by the third oscillator may be in the range of 200 MHz and the difference between the transmission frequency and the transition frequency is in the same range. It is then enough to work on the transition frequency to prompt changes, from one frame to another or periodically, in the transmission frequency and to maintain the required agility.
The problems of this type of operation are numerous. One of them is related to the great difference in frequency that exists between the different standards. Indeed, from the GSM standard to the DCS standard, the frequency is quite simply doubled. In addition, the frequency excursions used to achieve frequency agility, which are themselves standardized, are quite considerable. Thus, for the GSM standard, the dynamic range of frequency (in transmission or in reception) is 35 MHz. For the DCS and PCS standards it is 75 MHz and 60 MHz respectively.
For a single-band apparatus furthermore, one and the same transition frequency oscillator is used in the reception channel. In this case, the signal that it produces is mixed in a reception mixer with the received signal. The reception mixer then produces a signal whose frequency is substantially equal to the frequency of the intermediate frequency oscillator so as to enable immediate demodulation in base band. Thus, the number of mixers and the number of the demodulations are limited.
By way of an indication, a separation between the frequency band allocated to transmission and the frequency band allocated to reception is equal to 10 MHz in the GSM standard, 20 MHz in the DCS standard and in the PCS standard. This ultimately means that the transition frequency oscillators must be capable of a dynamic range respectively of 80 MHz (35 plus 10 plus 35), 170 MHz (75 plus 20 plus 75) and 140 MHz (60 plus 20 plus 60) for these three standards respectively. Given these very significant excursions (which are in the range of 10% of the nominal frequency of the oscillator), it does not appear to be possible, in the present state of the technology of manufacture of these oscillators, to make them cover two bands, even the two closest ones (DCS and PCS). To make them cover a single band is already a problem.
Indeed, for high quality service with a mobile telephone, the spectral purity of these oscillators has to be higher than xe2x88x9287 dBc/Hz to 10 KHz of the carrier, higher than xe2x88x92107 dBc/Hz to 100 KHz of the carrier and higher than xe2x88x92140 dBc/Hz to 3 MHz of the carrier. The frequency setting range of the oscillator at the transition frequency is theoretically between 1 and 2 GHz. In fact, the complexity of these oscillators, for GSM, DCS and PCS applications, leads to their being placed in three categories.
A first category lists the voltage-controlled oscillators known as non-adjusted oscillators. For these oscillators, only the controlled voltage makes it possible to compensate for the dispersion of characteristics of their components, and to work on a certain frequency band around an imposed base frequency. In the mobile telephones today, the supply voltage is about 3 volts. Indeed, they use three battery elements, giving 3.6 volts in rated voltage. A regulator reduces this voltage to three available volts. The useful controlled voltage is then between 0.5 and 2.5 volts. Given the compensation for the dispersion of the characteristics by the voltage, this voltage range cannot be exploited to the full extent and, in practice, the frequency band around an imposed base frequency can only be about 40 MHz. These first category oscillators are also the least expensive ones.
A second category lists the oscillators known as adjusted oscillators. For these oscillators, the dispersion of the characteristics due to their components is compensated for by preliminary settings. The controlled voltage is used however used to compensate for the temperature dispersions. In this case, for one and the same range of control voltage of 2 volts, it is possible to have a working range of 100 MHz around an imposed base frequency.
In a third category, the useful band is further increased around a base frequency imposed in two ways. Either a resonator of the oscillator is switched over or the range of control voltage is increased. The switching over of an element in the oscillator makes it possible to have a second band. Since this switched element is not adjusted, it is difficult to achieve high precision. Furthermore, if an element of this kind is very important, it adds a temperature drift in the oscillation frequency. For this reason, the frequency leap is limited. The two bands must remain within a limited band. For one and the same range of control voltage of 2 volts, there is thus obtained a range of operation of 220 MHz. Comparable results are obtained by using a voltage multiplier. The oscillator does not become more complex, but the control circuit for its part is complex. The oscillators of this third category are obviously far costlier than those of the second category which themselves are costlier than those of the first category.
There is a fourth category comprising two oscillators in one and the same pack. It is comparable, in terms of cost, to two oscillators of the second category.
Given these constraints of limited dynamic range of frequency, the growing complexity of these oscillators has a direct impact on their cost. It is thus possible schematically to assign them costs with a value 1, 2 and 3 depending on their category. Another criterion for appreciating the cost of a transmitter-receiver stage would be to add up all the frequency bands of all the oscillators involved.
A multiband mobile telephony telephone ought to have as many sets of transmission/reception systems as are desired in order that it be capable of covering the different bands. The cost of a mobile telephony telephone of this kind would then be directly proportional to the number of bands served. In general, its cost would be multiplied by four, especially because the oscillators would be multiplied. This is not acceptable.
According to the invention, this problem will be resolved on the basis of a limited number of oscillators. In a complete version, and in one mode of use, there will be only be five or even four oscillators available: namely even less than what would be necessary to permit operations in two distinct bands. The first object of the invention therefore is to diminish the number of voltage-controlled oscillators used and reduce their cost.
Another object of the invention, when oscillators are used with one and the same low setting range, is not to have to switch the oscillator over to the frequency of transition between transmission and reception, whereas the setting ranges of these oscillators are small as compared with the desired dynamic range of frequency variation for any given standard. Indeed, it is sought to be able to exploit all the possibilities of the standards which, when it is sought to transmit large quantities of information, stipulate transmission in successive temporal windows. In this case, the change from transmission to reception must be done during very short periods of time. Oscillator switch-over is ruled out in this case: the switch-over time of an oscillator with a wide dynamic range is far too long.
Thus, in the invention, it is seen to it that the dynamic range of control of the transition frequency oscillators may be low and that they do not have to be switched over. In this way, the oscillators are less expensive while at the same time meeting requirements.
One of the means of the invention consists in making frequency dividers available at output of the oscillators, especially at output of the intermediate frequency oscillator and also between the first mixer and a comparator. Then, action is taken on the division coefficients of these dividers so as to produce frequency combinations by algebraic composition (addition or subtraction) enabling the exploration of all the allocated bandwidths, in transmission and in reception, in furthermore limiting the dynamic setting range of a variable oscillator, essentially the transition frequency oscillator.
The solution of the invention is then noteworthy in that the oscillators used are not switched over when passing from transmission to reception and vice versa. Since there is no such switch-over, the build-up time to lead to a given use of an oscillator, due to a frequency agility, leads to the ability to use successive temporal windows in one and the same TDMA frame, which is favorable to increasing the information bit rates to be transmitted with a mobile telephony telephone.
Indeed, a radiofrequency synthesizer, the transition frequency oscillator, because of the principle of the heterodyne reception (selectivity filtering on a single intermediate frequency) must follow frequency steps of the input signal, namely channels staged in steps of every 200 KHz. This is not the case with the intermediate frequency oscillator which, especially in GSM, DCS and PCS, may be synthesized from a recommended clock frequency. The recommended frequency is 13 MHz and leads to a 1 MHz step obtained by division by 13.
Owing to its 1 MHz step and therefore its comparison frequency of 1 MHz, the intermediate frequency oscillator is faster (in a ratio that is the square root of the ratios of the comparison frequencies: in this case 5) than the transition frequency oscillator, this being the case for the same loop and for equivalent spectral purity values.
This speed enables the oscillators to be cut off for a longer time between each temporal window, which results in lower consumption. It also enables the use of several windows in one and the same frame for data transmission in accordance with the requirements of the GSM standard 05 02.
In the invention, the range of operation of the transition frequency oscillator is determined by choosing the criteria of the presence of the dividers, the abandonment of the switching over of the transition frequency oscillator and an infradyne/supradyne composition both in transmission-reception and in different GSM, DCS and PCS bands. This leads to low-cost solutions since the single transition frequency oscillator is in the third category while the intermediate frequency oscillator is in the first category. Or else, these two oscillators are, at most, in the second category. The result of this is a significant reduction of the price of transmitters-receivers and therefore of the multiband telephone.
An object of the invention therefore is a method for the setting of a multiband telephony receiver comprising a radiation antenna, a transmission channel and a reception channel connected to this aerial, a first, second and third voltage-controlled oscillator respectively delivering a signal at a transmission frequency, a signal at a transition frequency and a signal at an intermediate frequency, the signal at the transmission frequency being transmitted by the first oscillator to the antenna, a first mixer connected at input to the outputs of the first and second oscillators and receiving, from this first oscillator, the signal at the transmission frequency and, from this second oscillator, the signal at the transition frequency, a comparator connected at input to the output of the first mixer and to the output of the third oscillator, and delivering at output a control signal for the first oscillator, and between the first mixer and the comparator and, between the third oscillator and the comparator respectively, a first and a second frequency divider,
characterized in that it comprises the following steps:
a system of inequalities is set up taking account of a frequency-limited dynamic range of a single transition frequency oscillator so that this oscillator covers all the desired bands in transmission and reception,
this system of inequalities possessing unknown quantities that are frequency ranges of this transition frequency oscillator and this intermediate frequency oscillator,
the system of inequalities is resolved by choosing values of division of the frequency dividers below a predetermined number,
the transmitter-receiver is set with solutions of the inequalities by programming the dividers,
and the first mixer is made to work in infradyne mode for one frequency band and in supradyne mode for another.